New research shows that the rise in minimum temperatures is far above changes that can be attributed to natural climate variability – it is a manifestation of climate change

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Minimum temperatures are showing “more pronounced” changes across India, compared to the maximum, a new set of studies by researchers at the Indian Institute of Science (IISc), Bangalore, has found.

While the maximum temperatures showed “a significant upward trend” in the post-monsoon months across most of India, there was a rise in minimum temperatures in most seasons all over India, they say.

The researchers at the IISc studied changes in annual and seasonal maximum and minimum temperatures over India, from 1950 to 2005. The work was part of ‘detection and attribution’ (D&A) studies  that aim, through detailed statistical analysis and multiple lines of evidence, to confirm that a detected change in climate is due to warming caused by human activities and not due to natural variations seen in the climate.

“To date, formal D&A studies either for all over India or for any temperature homogenous regions of India is not available,” the IISc team says.

“Various studies have been carried out to detect and attribute the recent temperature changes in other parts of the world. Here (in India) for the first time, a D&A study for temperature change over India is attempted,” Dasika Nagesh Kumar, professor at IISc’s department of civil engineering, told indiaclimatedialogue.net.

“This study employs formal D&A analysis to extract the inscrutable details about observed climate behaviour.”

Previous studies – including one by Kumar’s team – had “detected” a faster increase in minimum temperature trend since 1970 compared to maximum temperature over most parts of India. The present study tries to find the reason for the trend.

The IISc scientists analysed temperature changes not only across India, but also in five sub-groups of ‘homogeneous’ zones with similar temperature characteristics – east coast, west coast, north central India, northwest India and interior peninsula.

They studied variations in minimum and maximum temperatures in four seasons – winter (January to February), pre-monsoon (March to May), monsoon (June to September) and post-monsoon (October to December).

The attempt was to see if the changes were above natural climate variations. For this, the researchers used the archives of the new generation of global climate model CMIP5 (coupled model inter-comparison project). It simulates historical climate data.

CMIP5 attempts to improve understanding of past, present and future climate changes arising from either natural variability or in response to external changes. It tries to evaluate how realistic different climate models are in simulating the recent past, provide projections for future climate change up to 2035 and 2100 and beyond; and understand why different models give different results.

The changes

Based on their analysis, the IISc scientists report that minimum temperatures show “more pronounced” changes, compared to maximum temperatures in India. While the maximum temperatures showed “a significant upward trend” in the post-monsoon months in most regions, there was a rise in minimum temperatures across India in most seasons. The number of “significant trends” was greater for minimum temperatures than for maximum.

Changes in minimum temperatures are above natural climate variability, says their report, which has now been accepted for publication in the Journal of Water and Climate Change.

The scientists cannot attribute the changes to any single cause.

The IISc findings are in line with observations by other Indian climate scientists.

Madhavan Nair Rajeevan, director of the Indian Institute of Tropical Meteorology (IITM), Pune, told a workshop on climate change and health, held in New Delhi in September, that the annual mean, maximum and minimum temperatures over India showed a “significant increasing trend of 0.6 degrees Celsius, 1.0 degrees C and 0.18 degrees C per 100 years, in IITM’s analysis of data from 1901 to 2010.” But when he looked at the period 1981 to 2010 – similar to the period studied by the IISc researchers – he found that the minimum temperature had risen far more significantly.

“On the whole, the rate of rise in annual mean temperatures is much higher since 1980s, mainly due to a sharp rise in the minimum temperatures,” he said. For the period 1981 to 2010, the mean, maximum and minimum temperatures rose at almost equal rates of around 2 degrees C per decade, “which is many times higher than for the whole period (1901-2010).”

Rajeevan said the frequency of occurrence of hot days during the pre-monsoon season shows a “significant increase” over the east and west coasts of India and interior peninsula. The frequency of hot nights has risen over the east and west coasts and northwest India. The frequency and duration of heat waves and heat stress over northwest India and the east coast have also increased by five days over the past 50 years.

Implications for water

The IISc researchers say that since climate drives the water cycle, “better understanding of changes in climate is essential for water resources management.”

Kumar’s colleague Sonali Pattnaik says, “In the past, most of the decisions related to water supply planning and management were based on a stationary climate, which does not hold good presently due to human-induced effects.”

Their findings may warrant policy changes, especially on hydro project design, water supply planning and water management.

“Temperature has a significant and direct influence on almost all hydrological variables,” Kumar explains.  Temperature and rainfall are the key factors that influence stream flow which, in turn, is an indicator of freshwater availability.

Lack of research

This is the first large-scale D&A study in India. There is still a lack of studies that can examine the correlation between climate change on one hand, and floods, droughts and storms on the other, though the Intergovernmental Panel on Climate Change had said these would be among the effects of global warming.

“Event-based attribution studies are useful to address risks and vulnerability,” Kamal Kishor Murari, from the centre for climate change and sustainable development at the Tata Institute of Social Sciences (TISS), told a workshop in July.

“In South Asia especially, because of the uncertainty in modelling and lack of baseline data, attribution of any damage and loss to climate change is difficult to make,” he said. Therefore, unlike the case of developed countries that are better able to attribute extreme events to climate change, in developing countries it would be “better to attempt to attribute seasonal events to climate change.”